The topographical distribution of duplication ends in the rIIB cistron is highly non-random, indicating sequence determination of location and frequency. Analysis of duplications in which rIIB function is provided by hybrid B cistrons indicates that the deletion r1589 fails to provide B function on E. coli TABR1 due to insufficient gene product rather than to an inactive product. It is anticipated that TABR1 will be useful in mapping regulatory elements of the rII region.